Wellbore percussion adapter and tubular connection

ABSTRACT

A percussion adapter that is driven to generate a percussive axial motion on a wellbore structure. One percussion adapter includes a drive connection to mechanically convert rotational drive to axially directed percussive motion. Another percussion adapter employs a valve that creates back pressure causing axially directed percussive motion. A wellbore tubular connection is also disclosed that transitions torque to resist back off when left hand or right hand torque is applied.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. application Ser. No.12/878,551 filed Sep. 9, 2010, and entitled “Wellbore Percussion Adapterand Tubular Connection.” U.S. application Ser. No. 12/878,551 is acontinuation-in-part of PCT application no. PCT/CA2009/000313 filed Mar.13, 2009, which claims priority from U.S. provisional application Ser.No. 61/036,328 filed Mar. 13, 2008, U.S. provisional application Ser.No. 61/076,050 filed Jun. 26, 2008, and U.S. provisional applicationSer. No. 61/138,017 filed Dec. 16, 2008. U.S. application Ser. No.12/878,551 claims priority from U.S. Provisional application Ser. No.61/266,462 filed Dec. 3, 2009. Each of the above listed patentapplications is hereby incorporated by reference in its entirety for allpurposes.

FIELD

The present invention relates to down hole tools and, in particular, awellbore percussion adapter for applying an axially directed percussiveeffect to wellbore structure and a tubular connection.

BACKGROUND

The application of a percussive force to a wellbore structure may be ofinterest. For example, if one could add a percussive force to the drillbit while drilling a wellbore, it is believed that the rate of drillingpenetration could be significantly increased, the required weight on bitcould be significantly reduced and torque required to turn the drill bitcould be significantly reduced. A “percussionized” drill bit should bean efficient drilling tool.

Many previous attempts at developing percussion adapters have focused onhydraulically driven devices. These devices use the flow of drillingfluid to drive pistons with a percussion adapter to create an axiallydirected percussive effect at the drill bit.

A common problem experienced in down hole operations relates to theeffect of torque on tubular connections. This problem may be exaggeratedwhen torque is generated in the operation of a tool down hole.

SUMMARY

In accordance with a broad aspect of the present invention, there isprovided a method for accelerating the drilling penetration of a rotarydriven drill bit, the method comprising: providing a positivedisplacement motor including a motor housing, a fluid discharge and arotor powered by fluid pressure; providing a drill bit; providing adrilling accelerator including a housing and a drive connection tomechanically convert rotational drive to axially directed percussivemotion; connecting the drilling accelerator below the motor includingconnecting the housing to move with the motor housing, connecting thedrive connection to be driven rotationally by the rotor and bringing thefluid passage into communication with the fluid discharge; connectingthe drill bit below the drilling accelerator with the drive connectionin drive communication with the drill bit; pumping fluid through themotor to drive the rotor and the drive connection to rotate and togenerate axial percussive motion which is communicated from the driveconnection to the drill bit and; discharging fluid from the fluiddischarge to pass through the drilling accelerator and The drill bit.

In accordance with another broad aspect of the present invention, thereis provided a drilling accelerator comprising: a housing including anupper end and a lower end; a drive connection including an upper axiallyrotatable drive shaft for receiving an input of rotational motion, arotational to axial mechanical drive converter in communication with theupper axially rotatable drive shaft for converting the input ofrotational motion to an axial sliding motion; a lower longitudinallymoveable drive shaft in communication with the rotational to axialmechanical drive converter to receive the axial sliding motion from therotational to axial mechanical drive converter and a lower drill bitinstallation site connected to the lower longitudinally moveable driveshaft for receiving the axial sliding motion and capable of conveyingaxial percussive motion there through, the lower drill bit installationsite telescopically mounted adjacent the lower end of the housing andslidably moveable relative thereto.

In accordance with another broad aspect of the present invention, thereis provided a method for applying an axially directed percussive forceto a wellbore structure, the method comprising: running into a wellborewith a string including (i) a positive displacement motor including amotor housing, a fluid discharge and a rotor powered by fluid pressure;(ii) a percussion adapter including a housing and a drive connection tomechanically convert rotational drive to axially directed percussivemotion; and (iii) a wellbore structure, the percussion adapter beingconnected below the motor such that the housing moves with the motorhousing, the drive connection is driven rotationally by the rotor andthe fluid passage is in communication with the fluid discharge and thewellbore structure being connected below the percussion adapter with thedrive connection in drive communication with the wellbore structure;pumping fluid through the motor to drive the rotor and the driveconnection to rotate and to generate axial percussive motion which iscommunicated from the drive connection to the wellbore structure and;discharging fluid from the fluid discharge to pass through thepercussion adapter toward the wellbore structure.

In accordance with another broad aspect of the present invention, thereis provided a percussion adapter comprising: a housing including anupper end and a lower end; a drive connection including an upper axiallyrotatable drive shaft for receiving an input of rotational motion, arotational to axial mechanical drive converter in communication with theupper axially rotatable drive shaft for converting the input ofrotational motion to an axial sliding motion; a lower longitudinallymoveable drive shaft in communication with the rotational to axialmechanical drive converter to receive the axial sliding motion from therotational to axial mechanical drive converter and a base end of thelower longitudinally moveable drive shaft for receiving the axialsliding motion and capable of conveying axial percussive motion therethrough, the base end mounted adjacent the lower end of the housing andslidably moveable relative thereto.

In accordance with another broad aspect of the present invention, thereis provided a percussion adapter comprising: an upper housing; a fluidflow channel extending from the upper end to the lower end of the upperhousing, the first fluid flow channel including a fluid entry endopening adjacent the upper end and a fluid exit adjacent the lower end;a lower housing telescopically installed for axially sliding motionrelative to the lower end of the upper housing; a second fluid flowchannel extending from an inlet end open at the upper end to a dischargeend opening at the lower end of the lower housing, the inlet end beingin fluid communication with the outlet end of the first fluid flowchannel; an axial drive generator operable to create a force to drivethe upper housing axially away from the lower housing such that theupper housing can be forced down against the lower housing to create ahammering effect against the lower housing.

In accordance with a broad aspect of the present invention, there isprovided a method for applying a vibratory force to a downhole string,the method comprising: providing a wellbore string with a percussionadapter installed therein, a fluid supply to the percussion adapter anda drill bit installed below the percussion adapter; positioning thebottom hole assembly relative to a formation to drill a wellbore;pumping fluid through the percussion adapter to generate axialpercussive motion by a driver causing an upper housing of the percussionadapter to lift away from and drop down on a lower portion of thepercussion adapter, the axial percussive force being communicated to thewellbore string to create a vibration therein and; discharging fluidfrom the percussion adapter to continue to pass through the wellborestring.

In accordance with another broad aspect of the present invention thereis provided a wellbore string tubular connection comprising: a firsttubular including a first threaded pin end with a right hand thread formand a protrusion extending from its pin end face to create a steppedregion thereon, a second tubular including a second threaded pin endwith a left hand thread form and a recess on its pin end face forming ashoulder sized to accept the stepped region of the first pin end seatedthereagainst, and a collar including a first threaded box with a firstselected thread form selected to threadedly engage the right hand threadform of the first threaded pin end and a second threaded box with asecond thread form selected to threadedly engage the left hand threadform of the second threaded pin end.

In accordance with a broad aspect of another invention, there isprovided a method for making up a wellbore connection, the methodcomprising: providing a first wellbore tubular with a threaded pin endand an tooth extending from a pin end face thereof, a second tubularwith a threaded pin end and recess in a pin end face thereof, the recesssized to accept the tooth of the first wellbore tubular and a collarincluding a first threaded box end and an opposite threaded box end;aligning the first wellbore tubular and the second wellbore tubular tobe threaded into the box ends of the collar and with the tooth alignedwith the recess and rotating the collar about its long axis to engagethe threaded pin ends of the first wellbore tubular and the secondwellbore tubular and draw the threaded pin ends into the collar.

It is to be understood that other aspects of the present invention willbecome readily apparent to those skilled in the art from the followingdetailed description, wherein various embodiments of the invention areshown and described by way of illustration. As will be realized, theinvention is capable for other and different embodiments and its severaldetails are capable of modification in various other respects, allwithout departing from the spirit and scope of the present invention.Accordingly the drawings and detailed description are to be regarded asillustrative in nature and not as restrictive.

BRIEF DESCRIPTION OF THE DRAWINGS

Referring to the drawings, several aspects of the present invention areillustrated by way of example, and not by way of limitation, in detailin the figures, wherein:

FIG. 1 is a schematic sectional view along a portion of a drill string.

FIG. 2 is an axial sectional view along one embodiment of a drillingaccelerator.

FIG. 3A is an axial sectional view along another drilling accelerator.

FIG. 3B is an elevation, partly in section, of a drive shaft useful in adrilling accelerator.

FIG. 4A is an axial sectional view along another drilling accelerator.

FIG. 4B is perspective view of a cam-type drive converter useful in thepresent invention.

FIG. 4C is an axial section through a cam-type converter useful in thepresent invention.

FIG. 4D is an axial section through a roller-type cam insert useful inthe present invention.

FIG. 4E is a front elevation of the insert of FIG. 4D.

FIG. 5 is an axial sectional view along another drilling accelerator.

FIG. 6 is an axial sectional view along another drilling accelerator.

FIG. 7A is a schematic illustration of a first embodiment of a bottomhole assembly.

FIG. 7B is a schematic illustration of a second embodiment of a bottomhole assembly.

FIG. 7C is a schematic illustration of a third embodiment of a bottomhole assembly.

FIG. 7D is a schematic illustration of a fourth embodiment of a bottomhole assembly.

FIG. 8A is an exploded, axial section through a tubular connectionuseful in the present invention.

FIG. 8B is an assembled, axial section through a tubular connectionuseful in the present invention.

FIG. 8C is a section along line I-I of a pin end of FIG. 8A.

DESCRIPTION OF VARIOUS EMBODIMENTS

The detailed description set forth below in connection with the appendeddrawings is intended as a description of various embodiments of thepresent invention and is not intended to represent the only embodimentscontemplated by the inventor. The detailed description includes specificdetails for the purpose of providing a comprehensive understanding ofthe present invention. However, it will be apparent to those skilled inthe art that the present invention may be practiced without thesespecific details.

A percussive adapter can be installed and operated to apply a percussiveforce to a downhole structure such as a drill bit, a liner, etc. Whenused for drilling purposes, percussive adapters are sometimesalternately called drilling accelerators, drilling hammers, and fluidhammers. A drilling accelerator can be installed in a drill string tofacilitate wellbore drilling operations. A drilling accelerator createsa percussive effect applied to the drill bit that alone or with rotarydrive of the bit causes the drill bit to drill into a formation.

With reference to FIG. 1, the lower end of a drill string is shown. Adrilling accelerator 10 can include a drive converter connection thataccepts rotational drive about axis x from a torque generating device 12above and converts that rotational drive to an axially directedpercussive force that is output to a bit box sub 14 positioned below thedrilling accelerator. When such a drill string is in use with a drillbit 16 installed in the bit box and the drill bit being rotationallydriven, arrow R, the axially directed percussive force, arrow P, appliedto the bit box sub is conveyed to the drill bit and can facilitatedrilling at the drill bit.

One embodiment of drilling accelerator 10 is shown in FIG. 2. Drillingaccelerator 10 may include an outer housing 18 including an upper end 18a and a lower end 18 b. Outer housing 18 is rugged, being exposed on itsexternal surface 18 c to the wellbore annulus and houses therewithin thedrive components for generating a percussive force.

To facilitate construction of the drilling accelerator, as will beappreciated, the housing can be formed in sections and connectedtogether by various means such as by welding, interlocks or threadedengagement, as shown at connections 21.

Upper end 18 a of the housing is formed for connection into a drillstring, such as by forming as a threaded connection. Lower end 18 b ofthe housing is formed for connection, shown herein directly but may beindirect, to a bit box sub 14. Bit box sub 14 has formed therein a site,such as, for example, threaded bit box 20, for accepting connection of adrill bit.

Bit box sub 14 is connected for rotational movement with housing 18through a splined connection 22. However, connection 22 permits axialsliding motion of the bit box sub within housing 18, such axial slidingmotion being generated by a connection to the drive connection ofdrilling accelerator 10. The drive connection is intended to drive thebit box sub axially to apply a percussive force at any drill bitconnected into the bit box during drilling. Seals may be provided, suchas O-rings and wiper seals 24 to resist fluid passage between thehousing and the bit box sub, etc.

In one embodiment, the drive connection includes an axial shaft 30supported in bearings 32 to convey rotational drive from an input end 30a to an output end 30 b which carries a bevel gear 34. This bevel gear34 meshes with a second bevel gear 36 mounted on transverse shaft 38which is rotatably supported in the housing. Transverse shaft 38includes an eccentric 40 thereon which drives a drive shaft 42. Driveshaft 42 includes a strap 44 with a bearing 46 therein in whicheccentric 40 rotates. Drive shaft 42 at its opposite end includes an eye48 through which the drive shaft is pinned via pin 50 to a percussionadapter 52 secured to bit box sub 14 for a drill bit.

Rotation in shaft 30 through reduction gears 34, 36 will impart on thepercussive adapter 52 an axially directed reciprocation determined bythe throw of eccentric 40. This axially directed reciprocation is thenconveyed directly to any bit secured in the bit box 20 of the bit boxsub.

The input torque may be generated by a mud motor. For example, axialshaft 30 may be connected to a rotor of a mud motor such that anyrotation of the rotor, by flow of drilling fluid through the motor, maybe conveyed to the drive connection. In one embodiment, the mud motormay include a positive displacement-type motor (PDM), which usespressure and flow of the drilling fluid to turn a rotor within a stator.Shaft 30 can be connected directly or indirectly to the rotor, asthrough threaded connection 60. Where a bent sub is positioned betweenthe motor and the drilling accelerator, a universal connector may bepositioned therebetween to convey rotation from the rotor to the axialshaft.

The fluid that drives the motor can continue down through theaccelerator and to the bit. As such, the accelerator may includedrilling fluid passages 54 that can be connected in communication withthe motor discharge and that extends from end 18 a, about the driveconnection components, to passages 62 through adapter 52 into a bore 56of bit box sub 14. Passages 54 may be formed, as by milling, etc.through outer housing 18 and can be directed by ports, seals, etc. fromthe discharge of the pump to passages 62 into the inner bore of the bitbox. In some embodiments, outer housing 18 may require thickening orlaminate/telescopic construction to accommodate the passages.

Gears 34, 36 and other moving parts may be grease packed for lubricationthereof. A compensator may be provided, for example, in end 30 a toaccommodate or alleviate pressure differentials which may occur duringdown hole operations

The embodiment of FIG. 2 operates to drill a borehole by applying apercussive force through the drill bit to the formation, with or withoutrotating the drill string from surface. In another embodiment shown inFIG. 3A, the drilling accelerator may include a drive system forconveying rotational drive from the motor to the drill bit in additionto the percussive forces generated thereby.

Referring, therefore to FIG. 3A, a drilling accelerator is shownincluding an outer housing 118 including a lower end 118 b, an axialshaft 130 (shown in part) to convey rotational drive from an input to agear transmission, a shaft 138 including an eccentric 140 for driving adrive shaft 142 and a percussion adapter 152 formed integral with a bitbox sub 114 for a drill bit.

In this embodiment, the gear transmission includes gears to convey bothrotational and axially reciprocal motion to the bit box sub. Asillustrated, for example, gear transmission includes a first gear 135that accepts input from bevel gear 134 and meshes with a gear 136 thatdrives shaft 138. Gear 136 also meshes with a second bevel gear 139 thatdrives the rotation of an inner housing 166. Inner housing 166 extendsand rotates within outer housing 118. Inner housing 166 is connected atits lower end for rotational transmission to bit box sub 114. Inparticular, as shown, bit box sub 114 is connected for rotationalmovement with housing 166 through a splined connection 122 such that anybit installed in the bit box can be driven to rotate by rotationconveyed from shaft 130.

Connection 122 also permits axial sliding motion of the bit box subwithin housing 166, such axial sliding motion being generated by aconnection to shaft 138 of the drilling accelerator, the shaft intendedto drive the bit box sub axially to apply a percussive force at anydrill bit connected into the bit box during drilling, while gear 139 andhousing 166 drive rotation of the bit box sub. Seals may be provided,such as O-rings and wiper seals 124 to resist fluid passage between thehousing and the bit box sub, etc. Outer housing 118 can extend down toprotect the inner housing. Bearings 168 and seals 124 a may be providedto facilitate rotation and seal against fluid and debris migrationbetween the parts.

As best shown in FIG. 3B, in this embodiment, drive shaft 142experiences differential rotation therealong: where upper portion 142 ais not rotatably driven, but lower portion 142 b is pinned to percussionadapter and is rotatably driven. In order to accommodate differentialrotation along shaft 142, a bearing 170 can be provided along itslength. Bearing 170 allows rotational motion therein of part 142 b aboutits long axis relative to part 142 a, but resists axial sliding motionsuch that axial percussive movement generated by the throw of eccentric140 is conveyed along the shaft rather than being absorbed.

The input torque may be generated by a mud motor. For example, axialshaft 130 may be connected to a rotor of a mud motor such that anyrotation of the rotor, by flow of drilling fluid through the motor, maybe conveyed to the drive connection.

It is to be understood that a cam and cam follower can be used toreplace an eccentric and connected drive shaft (i.e. items 40, 42 ofFIG. 2). When using cams, it may be useful to use weight on bit tomaintain the contact between the cam parts.

For example, as shown in FIG. 4A, another drilling accelerator 210 mayinclude an outer housing 218 including an upper end 218 a and a lowerend 218 b. Outer housing 218 is rugged, being exposed on its externalsurface 218 c to the wellbore annulus and houses therewithin the drivecomponents for generating a percussive force to be applied to a bitconnected therebelow.

To facilitate construction of the drilling accelerator, as will beappreciated, the housing can be formed in sections and connectedtogether by various means such as by welding, interlocks or threadedconnections 280.

Upper end 218 a of the housing is formed for connection at the distalend of a drill string. Lower end 218 b of the housing accommodates a bitbox sub 214, which telescopically extends from lower end 218 b.

Bit box sub 214 has formed therein a site, such as, for example,threaded bit box 220, for accepting connection of a drill bit (notshown). A bushing and safety catch 224 acts between housing 218 and sub214 to allow rotation of the sub within the housing and may secure thesub against fully passing out of the housing lower end 218 b. Safetycatch 224 allows some axial sliding motion of sub 214 within thehousing, such axial motion, for example, resulting from moving the subbetween a lower position (as shown) and an upper, weight on bit positionand being that as a result of the percussive force. In one embodiment,safety catch 224 may be eliminated with the safety provisions thereofinstead taken up entirely by interacting shoulders 225 a, 225 b on theparts. This allows housing end 218 b to be thicker along its length.

Bit box sub 214 is connected to an axial shaft 230, the combination ofsub 214 and shaft 230 acting to transmit drive energy from an input end230 a of the shaft to a drill bit installed in box 220. Bit box sub 214and axial shaft 230 may be connected by a telescoping splined connection222 that ensures continuous rotational drive conveyance while permittingaxial sliding motion of the bit box sub relative to shaft 230. Splinedconnection 222 may include a lubricating chamber that lubricatesinteraction between the shaft and the sub. Clearances in the splinedconnection may control the movement of lubricating fluid at the splinedconnection. As such, by selection of the tolerances at the splinedconnection, a shock absorbing feature may be provided which controls thespeed at which the parts can slide axially at the connection. Forexample, this may control the speed at which the parts can cometogether.

The input torque applied to end 230 may be generated by a mud motor. Forexample, axial shaft 230 may be connected to a rotor of a mud motor suchthat any rotation of the rotor, as by flow of drilling fluid through themotor, may be conveyed to the bit box sub. In one embodiment, the mudmotor may include a positive displacement-type motor (PDM), which usespressure and flow of the drilling fluid to turn a rotor within a stator.As will be appreciated, drilling fluid may be a liquid, gas, or acombination thereof.

Shaft 230 can be connected directly or indirectly to the rotor, asthrough threaded connection 260. Where a bent sub is positioned betweenthe motor and the drilling accelerator, a universal connector may bepositioned therebetween to convey rotation from the rotor to the axialshaft.

The fluid that drives the motor can continue down through the axialshaft and sub 214 and to the bit. As such, these parts may includedrilling fluid passages such as axial bores 254 passing therethroughthat can be connected in communication with the motor discharge.

Bearings 268 a, 268 a and bushings may be positioned between the axialshaft and the housing to accommodate radial and on bottom and off bottomthrust loads. A safety catch may also be provided between these parts.

Drilling accelerator 210 further includes a drive converter intended toconvert the rotational drive from the motor to an axial, reciprocatingmotion to drive the bit box sub axially to apply a percussive force atany drill bit connected into the bit box during drilling.

Referring now to FIGS. 4A-4C, in the illustrated embodiment, the driveconverter includes a pair of cam surfaces 270 a, 270 b. The first camsurface 270 a is installed in the housing and the second cam surface 270b is installed to move with bit box sub 214. Cam surfaces 270 a, 270 bare positioned to be separated by a gap 272 when the bit box is in itslower position, as shown, but can come together when weight is placed onbit. In other words, gap 272 closes when bit box sub is moved into itsupper, weight on bit position. Because the housing and shaft/bit box sub230/214 rotate at different speeds, the cam surfaces act to ride overeach other. Generally, rotation of sub 214 within and at a faster ratethan any rotation of the housing causes cam surface 270 b to ride overcam surface 270 a and cam surface 270 a effectively becomes the camfollower. Cam surfaces 270 a, 270 b include one or more cam protrusions274 a, 274 b that are oriented and configured to act with considerationof the direction of relative rotation therebetween such that the camsurfaces ride up over each other and drop down thereby generating anaxial percussive force to be applied to the bit box sub 214. Camprotrusions 274 a, 274 b have a ramped approach side, a peak and an exitside. The ramped approach side inclines upwardly to allow the camprotrusions to ride easily up over each other toward the peak. The exitside of the protrusions can be ramped down away from the peak, but amore significant percussive effect may be provided by forming the exitside as shown with an abrupt height change forming a drop off such thatthe forces (i.e. weight on bit) that drive the cam surfaces togetherforce the parts to abruptly close any gap between them, the gap formedwhen the protrusions exit off each other. The gap closing develops anabrupt, hammering vibration as the surfaces again come together. Whileone or more cam protrusions can be provided, it may be useful toposition the cam protrusions in a balanced fashion about surfaces 270 a,270 b, for example, by positioning the protrusions each equally spacedabout the circumference of the cam surface such that all protrusions areon the approach side at the same time. As shown, for example,protrusions 274 a, 274 b can be in pairs on each surface with a firstprotrusion of the pair diametrically opposed from the second protrusionof the pair on their cam surface. This may reduce adverse lateral forcesin the accelerator.

Cam surfaces 270 a, 270 b may be formed from materials that accommodateconsiderable wear without rapid break down.

In another embodiment, one or both of the cam surfaces may be includebearings to facilitate movement of the surfaces over one another andreduce detrimental wear to increase tool longevity. For example, in oneembodiment, as shown in FIGS. 4D and 4E, the cam protrusions on one ofthe cam surfaces, for example, surface 270 a of FIG. 4A may be replacedby a cam insert 275 a carrying rollers 279 on the cam surface 270 c.Rollers 279 are installed to ride up over the cam protrusions 274 b ofthe opposite surface (i.e. surface 270 b) and drop down the exit side ofthe protrusions to create a vibratory effect. In one embodiment, therollers may be ball bearing type rollers carried in the selected camsurface. Alternately, the rollers may be cylindrical rollers, as shown,or conical type rollers held to rotate along an axis extending radiallyfrom the tool long axis X.

Rotation in shaft 230 and bit box sub 214 relative to housing 218 willimpart on sub 214 axially directed reciprocation determined by the throwof cam protrusions 274 a, 274 b of surfaces 270 a, 270 b. This axiallydirected reciprocation is then conveyed as a vibratory effect to any bitsecured in, directly or indirectly, the bit box 220 of the bit box sub.

The vibratory effect may be created by axially reciprocating movementcreated at the interacting cam surfaces which causes a hammering effectwhen the two parts impact against one another. However, in oneembodiment, the tool may be selected to create the vibratory effect byfirst generating axially reciprocating movement at the interacting camsurfaces that in turn cause a hammering effect at surfaces apart fromthe cam surfaces. In such an embodiment, the form of the cam surfacesmay be preserved by reducing the detrimental wear caused by the partsstriking against one another. In particular, while the reciprocatingaction is generated at the cam surfaces, the impact creating thehammering effect is generated elsewhere. Such an embodiment may beprovided, for example, by provision of a two part mandrel, as providedby axial shaft 230 and bit box sub 230, selected to take up and generatethe hammering effect caused by the throw of the cam surfaces. In theillustrated embodiment, for example, while the cam surfaces 270 a, 270 bcreate an axially reciprocating motion, the hammering effect generatedby that motion occurs at the telescoping splined connection 222. Theaxially sliding motion that is created by the cam surfaces riding overone another causes the upper cam surface 270 a, housing 218 and axialshaft 230 to be raised relative to bit box sub 214, as by axial movementbetween axial shaft 230 and bit box sub 214 at the telescoping splinedconnection 222. As the cam surfaces continue to ride over one another,the cam protrusions 274 a, 274 b (which may or may not include rollers)will drop off each other on their exit sides and this, in turn, causesupper cam surface 270 a, housing 218 and axial shaft 230 to drop down.When this happens, end 230 a of shaft 230 will strike against upper end214 a of bit box sub 214 (inside connection 222) creating a hammeringeffect that is conveyed to the bit in bit box 220. To ensure that themajor striking force occurs at connection 222 between parts 214 a and230 a, any operational gap between parts 214 a, 230 a, which is themaximum gap distance achieved when there is weight on bit driving sub214 up into the housing and the cams have driven parts 214 a, 230 aapart, should be at least slightly less than the maximum, unrestrictedthrow of cam surfaces, which is the maximum unrestricted distance thatcould be traveled by upper cam surface 270 a as its cam protrusions 274or rollers drop off the cam protrusions or rollers on lower cam surface270 b. If the gap between parts 214 a and 230 a is more than the throwof the cam surfaces, the cam surfaces will strike each other before theaxial shaft and bit box sub can come together. Although this will createa percussive effect, it does cause greater wear at the cam surfaces andrequires the use of adequate thrust and radial bearings along the axialshaft. However, by selecting the gap distance between parts 214 a, 230 ato be less than the throw of the cam surfaces, the hammering is taken upand generated along the shaft, which maintains the force in line,concentrated around the center axis x of the tool and between morerugged parts. In such an embodiment, the cam surfaces also are protectedfrom at least some wear, reducing their need for repair or replacement.

In one embodiment, the contact surfaces between parts, where thehammering effect is generated may be supplemented with percussion platesthat have a greater wear resistance than the other materials of theseparts. In one embodiment, seals or structures may be provided tofacilitate fluid flow through bore 254 past the impact area betweenparts 214 a, 230 a. For example, in one embodiment, a sleeve/nipple mayprovided on one part 214 a or 230 a that inserts into an enlarged regionof the bore formed on the other of the two parts and seals, such aso-rings may be provided therebetween to prevent fluid from passing frombore 254 into the impact region between the parts. The splinedconnection 222 may also provide a cushioning effect. As noted above, theclearances in the splined connection may be selected to cushion themovement of the parts when the gap is closing such that shock generationis controlled. In one embodiment, the clearances at the splinedconnection are selected to control the degree to which the axialmovement is cushioned at the connection and such that a hammering effectis generated but the shock generation is controlled.

In the illustrated embodiment, first cam surface 270 a is provided by aring 275 installed in housing 218. Ring 275 forms surface 270 aannularly with protrusions 274 a downwardly facing. A bore 276 in thering provides an opening through which a portion of shaft 230 (asillustrated) or bit box sub 214 extends. Second cam surface 270 b, inthe illustrated embodiment, is provided by a ring 277 that includesthreads 278 for securing on an end of sub 214 such that surface 270 b isfacing upwardly to position its cam protrusions 274 b for engagementagainst those on surface 270 a.

Ring 275 and housing 218, at shoulder 218 c, bear against each othersuch that movement, such as upward movement caused by interaction of thecam surfaces, is transferred to the housing. In addition, ring 275 andhousing, at shoulder 230 c, also may bear against each other such thatupward movement caused by interaction of the cam surfaces is as welltransferred to the shaft 230.

The embodiment of FIG. 4A operates to drill a borehole by applying apercussive force through the drill bit to the formation when weight isapplied on bit. When the bit is lifted off bottom, the bit box sub 214is able to drop into its lower position which separates the cam surfacesand discontinues the percussive force.

When weight on bit is resumed and axial shaft 230 is driven to rotate,cam surfaces 270 a, 270 b will be rotated at different speeds such thattheir cam surfaces will ride up over one another and drop off the exitside causing housing 218 and axial shaft to be lifted away from bit boxsub 214, as that sub and the bit it carries remains on bottom, and,thereafter, as the cam protrusions exit off one another, the housing andthe axial shaft drop down. When the housing and axial shaft 230 dropdown, a hammering effect is applied to bit box sub, as by surface 230 astriking surface 214 a.

The embodiment above operates effectively by lifting an upper part ofthe drilling accelerator, the housing 218, relative to a lower portion,in that embodiment, the bit box sub 214, to separate the parts and onceseparated, allowing the upper part to drop down on the lower part tocreate a hammering effect. This creates a vibratory force on the lowerpart, but tends to avoid the creation of harmonics, lateral loads andother adverse effects. In the foregoing accelerator, the lifting anddropping of the parts is driven by the interaction of cam surfaces. Inother embodiments, the lifting and dropping may be driven by otherparts. For example, in one embodiment a valve and pressure chamber maybe employed to drive the hammering effect. A valve may be employed thatcreates a seal against fluid flow at lower pressures, but which seal canbe overcome by fluid pressures above a particular pressure. As such, thevalve can work with fluid passing through the accelerator to cause fluidto build up in a chamber between the upper and lower portions of theaccelerator, such fluid build up causing the upper portion to be liftedrelative to the lower portion to create a gap between impact surfacesthereof and when the selected pressure is reached, the valve opens toallow the fluid to pass, thus releasing the pressure holding the partsapart and allowing the upper part to drop down on the lower part.

In one embodiment, a percussion adapter includes an upper housing, afluid flow channel extending from the upper end to the lower end of theupper housing, a lower housing telescopically installed for axiallysliding motion relative to the lower end of the upper housing, a secondfluid flow channel extending from an inlet end open at the upper end toa discharge end opening at the lower end of the lower housing, the inletend being in fluid communication with the outlet end of the first fluidflow channel, a valve to meter fluid passing from the first fluid flowchannel through the second fluid flow channel to create a back pressuredriving the upper housing longitudinally outwardly from the lowerhousing when the valve is closed and allowing the parts to come togetherwhen the valve is opened, the valve being opened by a fluid pressurewhen the pressure exceeds a set value and the valve being closed tocreate the back pressure when the pressure falls below the set value.

In one embodiment, for example, as shown in FIG. 5, another drillingaccelerator 410 may include an upper housing 418 including an upper end418 a and a lower housing 414 including a lower end, in this embodimentshown as a bit box 420. Housing parts 418, 420 are rugged, being exposedon their external surfaces to the wellbore annulus. Lower housing 420and upper housing 418 are telescopically connected such that lowerhousing 420 remains connected to the upper housing but can slide axiallyrelative thereto.

To facilitate construction of the drilling accelerator, as will beappreciated, the housing can be formed in sections and connectedtogether by various means such as by welding, interlocks or threadedconnections 480.

Upper end 418 a of the housing is formed for connection to a drillstring. Bit box 420, in this illustrated embodiment, is threaded toaccept connection, directly or indirectly, of a drill bit (not shown).In other embodiments, lower housing 414 may include other structuressuch as a liner connector, or a sub connection, rather than a bit box.

A fluid passage extends down through the drilling accelerator. The fluidpassage includes a bore 454 (shown in phantom) through the upperhousing, which opens at the upper end in communication with the fluidsupply from above (from a motor discharge for example) and opens at alower outlet end 454 a, and a bore 455 a, 455 b (shown in phantom) thatextends from the upper end of housing 414 to the bit box 420. As such,the fluid that is supplied during drilling can continue down through theupper housing and housing 414 toward the bit. Seals or structures may beprovided to contain fluid in bores 454, 455 a, 455 b avoiding pressureleaks between the parts 418, 414.

Drilling accelerator 410 further includes a driver to impart an axial,reciprocating motion to apply a percussive force at any drill bitconnected into the bit box during drilling.

In the illustrated embodiment, the driver acts to lift the upper housingtelescopically away from the lower housing and thereafter allows theupper housing to drop down on the lower housing to impact it and createa hammering effect thereon. The dropping motion of the upper housingrelative to the lower housing can be by gravity or weight on bit, whilethe lower housing remains restrained in the wellbore as by restingagainst bottom hole. In the illustrated embodiment, the hammering effectis generated at chamber 415, which is the gap that is opened up when theupper housing and the lower housing telescopically separate.

In this illustrated embodiment, the driver includes a valve 470 in fluidpassage 455 that controls fluid flow through the passage depending onthe pressure differential between pressure P1 in passages 455 a, 454 andin chamber 415 uphole of the valve and pressure P2 downhole of thevalve, in passage 455 b. The valve, for example, may be of a pressuresensitive type that remains closed at lower pressures but opens when thepressure reaches a certain, higher overcoming pressure. Various valvesare useful such as a poppet valve, a biased ball valve, etc. When valve470 remains closed, it causes pressure P1 to build up in passages 455 a,454 uphole of the valve. This build up in pressure causes the upperhousing and lower housing to be driven apart, to expand the volume ofchamber 415, thus in normal operation, lifting upper housing away fromthe lower housing. This expansion of chamber 415 continues until thevalve opens, after which the fluid is allowed to pass the valve and thepressure holding the parts 418, 414 apart is dissipated to close the gapbetween them.

The valve may be selected that any pressure dissipation is abrupt suchthat the upper housing can drop abruptly on to the lower housing. Thesudden release of pressure from chamber 415 allows the upper housing torapidly close the gap, such that the upper housing strikes against thelower housing, creating a hammering effect. The gap closing develops anabrupt, hammering vibration as the surfaces come together. When thepressure is released, valve 470 again closes and once again passages 454and 455 a and chamber 415 above the valve can begin to pressure up todrive the upper housing away from the lower housing. As such, the cycleto generate a subsequent hammering impact between the parts is againinitiated.

The hammering effect is generated along the long axis x of the tool,which maintains the force in line, concentrated around the center axis xof the tool and between rugged parts. For example, the valve 270 can bespaced away from the point of impact such that it and/or the valvehousing are isolated to some degree from the force generated.

The housings can take various forms. For example, in the illustratedembodiment, upper housing 418 includes three main parts a main bodydefining upper end 418 a, a shaft 418 d extending from the main bodyalong long axis x and a sleeve 418 e extending from the main body at thesame end as the shaft. Sleeve 418 a is spaced from shaft 418 d, butextends substantially concentrically about and alongside it such as thatan annular space is formed between the parts. In the illustratedembodiment, the upper end, the shaft and the sleeve are fixed togetherto move axially as one.

Considering the illustrated form of upper housing 418, lower housing 414in the illustrated embodiment, includes a bore 414 c extending into itsupper end. Lower housing 414 is telescopically positioned in the annularspace between sleeve 418 e and shaft 418 d with the shaft extending intobore 414 c. Lower housing 414 is retained by interacting shoulders 418c, 414 b from fully passing out of the end of sleeve 418 e. A bushing424 acts between housing 418 and housing 414 to reduce wear therebetweenand provide a good base for seals.

As the lower housing moves telescopically relative to the upper housing,shaft 418 d slides in bore 414 c. In the illustrated embodiment, ashoulder 414 d is formed in bore 414 c at which the diameter of the boreis reduced. Shoulder 414 d limits the advancement of shaft 418 d intothe bore. Chamber 415 is formed between the shaft and the shoulder andthese parts form the surfaces at which the hammering effect is, at leastin part, generated. The axial movement of upper housing relative to thelower housing causes chamber 415 to be opened and closed between shaft418 d and shoulder 414 d of bore 414 c. In particular, in one position,when shaft 418 d is driven against shoulder 414 d, chamber 415 iseffectively non existent defining no volume and, in another position (asshown), shaft 418 b is withdrawn from contact with the shoulder andchamber 415 has a volume.

In the illustrated embodiment, fluid passage 454 extends through shaft418 d. While the upper housing could be formed in other ways, in oneembodiment, shaft 418 d is formed by a mandrel that extends from orthrough (as shown) an axial bore 431 through the housing. In such anembodiment, fluid passage bore 454 extends through or alongside themandrel. Likewise, the lower housing may be formed in various ways, forexample, to facilitate installation of valve 470, a shaft 471 and insertbody 473 accommodating valve 470 may be installed in bore 414 c belowthe shoulder. In such an embodiment, fluid passage bore 455 a extendsthrough the shaft 471 to communicate to the valve and passage 455 bextends through body 473 below the valve.

Seals 422, 423 or other structures may be provided to contain fluid inbores 454, 455, avoiding pressure leaks between the parts 418, 414.

In the illustrated embodiment, the valve acts to lift the upper housingincluding shaft 418 d away from shoulder 414 d. Once an opening pressureis reached across the valve, the valve opens and the shaft of the upperhousing is driven by gravity or weight on bit against the shoulder inthe bore to create the hammering effect on the lower housing.

Impacting surfaces of shaft 418 d and shoulder 414 d may be formed frommaterials that accommodate considerable wear without rapid break down.In one embodiment, the contact surfaces between parts, where thehammering effect is generated may be supplemented with percussionplates, such as plate 417, which have a greater wear resistance than theother materials of these parts.

While a hammering effect is described as being generated about chamber415, hammering effects can be generated in addition or instead betweenother surfaces of the upper housing and the lower housing. For example,an external shoulder of lower housing can be contacted with an internalshoulder 418 e of upper housing to create a hammering effect, themovement between the parts being generated by valve 470.

The embodiment of FIG. 5 operates to drill a borehole by applying apercussive force through the drill bit to the formation when weight isapplied on bit. When the bit is lifted off bottom, the lower housing isable to drop into a lower position which separates the shaft from theend of the bore and substantially discontinues the percussive force, asit is mostly dissipated by shouldering of the lower housing. Inparticular, the valve may continue to cycle between an open and closedposition, but the lower housing will already be shouldered, with 414 bagainst 418 c, at a fully extended position with cavity 415 at a maximumvolume.

When weight on bit is resumed and fluid is driven into the flow passagesof the tool, the valve will begin to cause a pressure fluctuation thatcauses a pressure build up to lift the upper housing away from the lowerhousing and then to open the valve, when a selected pressure is reached,such that the upper housing can drop down on the lower housing. When theupper housing drops down, a hammering effect is applied to lower housingas by shaft 418 d striking surface 414 d. The hammering effect isconveyed to the bit box.

As will be appreciated, to facilitate the drilling operation, any bit inbit box 420 may be rotated. In the embodiment, illustrated in FIG. 5,the accelerator is intended to be rotatably driven by rotation of thedrill string. As such, the parts are secured as by splined connections423 and by corresponding faceting of shaft 418 d and bore 414 c formovement as a single unit.

In the embodiment of FIG. 6, the percussive adapter operates by use of avalve 570 that is capable of creating a back pressure that lifts upperhousing 518 away from lower housing 514 and thereafter opens to releasethe back pressure to allow the parts 514, 518 to come together creatinga sudden hammering impact. This is similar to the operation of the toolof FIG. 5. However, unlike that of FIG. 5, mandrel 530 acts to transmitdrive energy from an input end 530 a thereof to lower housing 514 and,therethrough, to any drill bit installed in box 520. For example, lowerhousing 514 and mandrel 530 may be connected to transmit torquetherethrough by a telescoping faceted connection 523 that ensurescontinuous rotational drive conveyance while permitting axial slidingmotion of the lower housing relative to mandrel 530.

The input torque applied to end 530 a may be generated by a mud motor.For example, mandrel 530 may be connected to a rotor of a mud motor suchthat any rotation of the rotor, as by flow of drilling fluid through themotor, may be conveyed to the bit box sub. In one embodiment, the mudmotor may include a positive displacement-type motor (PDM), which usespressure and flow of the drilling fluid to turn a rotor within a stator.The motor, in one embodiment, may include stator housing having a squarecross sectional shape, to facilitate increased weight on bit withflexure in the motor housing. Mandrel 530 can be connected directly orindirectly to the rotor, as through threaded connection 560. Where abent sub is positioned between the motor and the drilling accelerator, auniversal connector may be positioned therebetween to convey rotationfrom the rotor to the axial shaft.

Bearings 568 a, 568 d and bushings may be positioned between the mandreland the housing to accommodate radial and on bottom and off bottomthrust loads. A safety catch may also be provided between these parts.

Also, while the hammering effect is received at shoulder 514 d of lowerhousing, the contacting surface of upper housing is a shoulder 518 f ofmandrel 530. Mandrel 518 d includes a nipple end 518 d′ that extendsbeyond shoulder 514 d into bore 514 c and pressure chamber 515 isaxially distanced from the parts 514 d, 518 f at which the hammeringeffect is generated. As such, the possibility of drilling fluidcontamination of the shoulders. The distance between the upper housing(shaft) and the lower housing (valve housing) at chamber 515 is greaterthan between shoulders 514 d, 518 f so that the hammering effect doesnot occur adjacent the valve and the valve is protected from impactdamage.

While the foregoing describes the operation of various drillingaccelerators with a drill bit in a bit box sub, it is to be appreciatedthat the percussion adapters, rather than creating a vibratory effect ona bit, may of course be employed as agitators to apply a vibratory forceto other downhole structures. For example, a percussion adapter can beused as an agitator in a drill string for example spaced apart along thedrill string to create uphole vibration in the string. Such vibrationmay reduce drag. In such an embodiment, the bit box may be replaced witha drilling tubular connector. In another embodiment, the percussiveadapter may be secured above a liner to apply a vibratory effect tothereby facilitate installation thereof. In such an embodiment, the bitbox may be replaced with a liner connector, such as a releasable linerhanger. The percussive adapter may be secured above the liner and may beoperated to vibrate the liner into a selected position in the hole.Thereafter, the connector may be operated to position the liner in thehole, if desired, and the percussive adapter may be removed therefromand tripped to surface.

A percussive adapter according to an embodiment, generates a vibratoryeffect that facilitates drilling or other downhole operations butappears to avoid the generation of problematic harmonics and shocks. Itis believed that the lower frequency operation, short stroke length andcushioned movements may offer a more controllable, less harshenvironment, when compared to previous drilling hammers. For example,the fluid cushion provided at the splined connection, such as connection222 in FIG. 4A, and in particular the speed at which the lubricatingfluid can move past the spline components when they are moving, maycontrol the speed at which the parts can come together. Alternately orin addition, the operation of the hammer of FIG. 4A tends not to createmud pressure fluctuations, which may tend to avoid interferences. Assuch, while it may not have previously been thought to be possible withdrilling hammers, the present adapter can be employed with variousdownhole devices such as non-percussive drill bits, shock subs, surveytools and/or rotary steerable devices.

For example, while drilling hammers have often been employed withpercussive, non-standard drill bits, in one embodiment, the presenthammer may be operated with a standard bit, such as a tricone or PDC(polycrystalline diamond compact) bit. In one embodiment, for example, aPDC or tricone bit may be employed that is useful for steerableoperations.

Thus, in one embodiment, the present percussive adapters may be employedto drill in vertical holes, as well as in non-vertical holes such ashorizontal, deviated, lateral, and/or tangential boreholes and, in fact,through build angles.

In one embodiment, the present percussive adapters, for example oneaccording to FIG. 4A, tends to operate without interference with surveytool instrumentation which is normally sensitive to physical abuse. Assuch, tools with sensitive mechanical, electromechanical and/orelectronic components such as MWD, LWD, EM, pulse, gyroscope, moneland/or UBHO tools may be employed in a bottomhole assembly with thepercussive adapter. These tools may also be employed with the percussiveadapters of FIG. 5 or 6, if mud flow rate and pressure fluctuations canbe tolerated. Again, this permits the use of the tool in non-verticalboreholes.

In another embodiment, the present percussive adapters, for example,those of FIG. 4A, 5 or 6, tend to operate without interference of toolsoffering steerability. For example, the percussive adapters may beemployed with a rotary steerable tool. For example, the percussiveadapter of one of the foregoing embodiments of FIG. 4A, 5, or 6 may beemployed with a rotary steerable tool that includes a biasing memberthat biases the string to the high side of the hole to direct the drillbit in that direction. Such a rotary steerable tool may permit thestring to be rotated while the tool remains set in a biasing position.Such tools may be employed in a bottom hole assembly with the percussiveadapter. As such, again, this permits the use of the tool to formnon-vertical boreholes.

In another embodiment, a shock sub may be employed in a bottom holeassembly with the percussion adapter. A shock sub provides protectionagainst bottom hole assembly damage from vibrational shock. The shocksub may be installed above the percussion adapter at least at andpossibly higher than the neutral point considering weight on bitrequirements. The neutral point considers the point at which the stringtransitions from compression to tension. The position of the shock submay depend on the weight on bit required for operation of the bit and todrive the upper housing to drop down onto the lower housing and theweight on bit afforded by the BHA below which the shock sub ispositioned. For lighter weight on bit requirements, it may be useful toinstall the shock sub closer, for example, directly above the percussiveadapter. However, if the drilling operation is to proceed with highweights on bit (>15,000 Decs), then the shock sub may be positionedhigher up to reach the neutral point. If a shock sub is employed withsurvey tools, it may be useful to position the shock sub between thesurvey tools and the percussive adapter. However, as noted above, thepresent percussive adapter may not, in any event, interfere with thesensitive components of the survey tools.

FIGS. 7A-7D show various possible strings and bottom hole assembliesemploying the percussion adapter of the present invention.

The string 601 of FIG. 7A, for example includes a bottom hole assemblyincluding drilling accelerator 610 a adjacent a pdc (polycrystallinediamond compact) bit 674. The bit and accelerator are driven by a motor675 to drill a borehole 602 through a build angle, as monitored by asurvey tool 676. The string includes a second drilling accelerator 610 bpositioned uphole of the bottom hole assembly, which acts to inputvibrational energy to the drill string to counteract drag.

The string 701 of FIG. 7B, for example, includes a bottom hole assemblyincluding a drilling accelerator 710 adjacent a bit 774, which may forexample be of the tricone or pdc types. The bit and accelerator aredriven from surface, as by string rotation, to drill a borehole 702through a build angle, as driven by a rotary steerable tool 775 andmonitored by a measuring-while-drilling tool 776.

The string 801 of FIG. 7C, for example includes a bottom hole assemblyincluding a drilling accelerator 810 adjacent a bit 874. A shock sub 877a is positioned along the string to absorb and limit transmission of thepercussive forces generated by the drilling accelerator, as well a othernormal shocks associated with drilling along the string. Depending onthe neutral point of the string, as described above, the shock sub couldbe moved closer to the drilling accelerator, as shown in phantom at 877b.

The string of FIG. 7D, is for placement of a liner 978, rather than fordrilling per se. The string includes an upper work string 903 forsupporting the liner and manipulating it from surface, a liner hanger979 for securing the liner in the wellbore 902 once it is in positionand a percussive adapter 910 for imparting a vibratory energy to theliner to facilitate its installation into the well. For example, thepercussive adapter may apply a percussive effect to the liner tocounteract drag.

The use of a percussive adapter to apply a percussive, axially directedreciprocation to a drill bit may generate left hand torque in the drillstring. Such torque may adversely effect standard threaded connectionsalong the string, such as connections 280, causing them to becomeloosened or to unthread completely. As such, with reference to FIGS. 8Aand 8B, a threaded connection can be used at connection 280 or inconnections in other string components that can accommodate left handtorque substantially without weakening the connection. In oneembodiment, such a connection includes a collar 380 including a pair ofthreaded box ends 382, 384. Box end 382 includes a thread form extendingin a direction opposite from the thread form of box end 384. Forexample, if box end 382 includes a left hand thread, box end 384includes a right hand thread. As will be appreciated by persons skilledin the art of wellbore tubular strings, a collar is the term used todescribe a substantially cylindrical connector that is formed to acceptthreaded engagement of a pair of tubulars, each with a threaded pin end.The box ends each have thread forms that start adjacent the collar endface 382 a, 384 a, respectively, and extend fully or partially toward acrest 385. Crest 385 may be threaded or smooth, depending on the type ofcollar.

The illustrated connection further includes a first wellbore tubular 386and a second wellbore tubular 388, each formed with a pin end 386 a, 388a, respectively. Tubulars 386, 388 can by housing sections of a drillingaccelerator, mud motor, or other tubular portions of a down holeassembly or drill string. Each pin end has a pin end face 386 b, 388 b,respectively. The pin ends each include a thread form selected to threadinto their respective box end 382 or 384. Pin ends 386 a, 388 a furtherhave corresponding stepped regions formed by axial extensions from theirpin end faces such that the pin ends can engage each other to restrictor possibly eliminate relative rotational movement therebetween abouttheir long axis .chi.t, when they are held end face adjacent to end facein collar 380. The stepped regions are formed by varying the pin end'slength from its pin base to its end face, creating an axially extendingstepped area along the pin end face. For example, one pin end face 386 bincludes a stepped extension where the face has a length change creatinga shoulder 389 a while the other pin end face 388 b includes a steppedrecess along its circumference also creating a shoulder 389 b, thestepped recess is formed to correspond to and, for example, follow inthe reverse, the stepped extension of the first pin end such that thetwo shoulders can be seated against each other, preventing the two pinends from rotating relative to each other. To most effectively preventrelative rotation between the pin ends, the stepped regions may beformed of abrupt length changes creating sharper corners, rather thanbeing curved undulations that could ride over each other. Also, to allowthe pin ends to mesh, as by being advanced towards each other alongtheir long axis, it will be appreciated that the stepped regions mayform the shoulders along a line substantially aligned with the tubular'slong axis.

It will be appreciated that such shoulders formed by stepped regions andrecesses, form at least one tooth 390 a, 390 b extending from each pinend face 386 b, 388 b, each formed so that the pin end faces can meshand be prevented from rotating relative to each other.

The shoulders may be positioned to resist the relative rotation that isadverse to the threaded condition of the connection. For example, in oneembodiment, the shoulders may be positioned to provide resistance toback off by left hand torque. Alternately, each pin end face may includeat least one left hand facing shoulder and at least one right handfacing shoulder such that the tubulars are substantially prevented fromrotating in either direction relative to each other. In the illustratedembodiment, the tubulars each include a plurality of left and right handfacing shoulders forming, in effect, a plurality of teeth with gaps gtherebetween. The teeth on the first wellbore tubular are formed to meshclosely between the teeth on the second wellbore tubular. In particular,the teeth 390 a of the first tubular are formed to fit tightly betweenthe teeth 390 b on the second tubular such that, if the pins are broughttogether, end to end, the teeth 390 a fit into the gaps between teeth390 b with the sides 390 a′ of teeth 390 a positioned closely alongsidethe sides 390 b′ of teeth 390 b. In this position, engagement betweenthe shoulders 389 a, 389 b formed by the sides of the teeth preventsrotation of one pin end relative to the other, when they are held pinend to pin end in the collar.

Forces tending to urge the pin ends to rotate about their long axis tounthread from the connection are resisted by contact between theshoulders of the pin ends. As such, it is useful to provide a reasonablesurface area for contact between the shoulders of opposite pin ends. Inone embodiment, for example, corresponding shoulders may have sides 390a′, 390 b′ that are cut substantially radially, in other wordssubstantially along a radial line extending out from the center axis ofthe tubular.

Pin end faces and shoulders may have close tolerances. If some flex isdesired at the connection, such that the lateral rigidity at theconnection is reduced, tolerances may be relaxed between pin end faces,such that the length of the shoulder extension on one tubular does notquite equal the depth of the shoulder on the opposite tubular. In otherwords, the length L of the teeth, measured from tip 392 to base 393(FIG. 8C) on one tubular is more than the length of the teeth on theother tubular. The gap formed between the tips of one tubulars teeth andthe bases of the other tubulars teeth allows some lateral flex at theconnection. Another option to provide for more lateral deflection at theconnection, in addition or alternately to the foregoing, may be toindent the outer surface of the teeth. This reduces the thickness t(FIG. 8C) of the pin end along the length of the teeth and may create aspace between the tooth and the inner surface of the collar, when thepin is threaded into the collar. The surface indentation can beinitiated at a tapering surface 391 adjacent the base 393 of the teethto provide more landing space for lateral deflection.

The type of thread form, including for example, taper and pitch, used inthe connection is not particularly important. In one embodiment, amodified Acme thread may be used to enhance seating and to deter fluidmigration through the threaded interfaces at the connection, but otherthread forms may be used, as desired.

Seals may be provided in the connection, such as for example o-rings 392in the collar at the crest and/or at interfacing surfaces, for examplesurfaces 394 a, 394 b with close tolerances, to enhance the fluidsealing properties of the collar.

To make up the connection, the first and second wellbore tubulars arealigned to be threaded into their respective box ends of the collar andalso, the tubulars are aligned with their teeth offset so that the teethof each tubular are aligned to mesh into the openings between the teethof the other tubular. In this way, the stepped regions formed by theteeth on one tubular may be set against the shoulders formed by theteeth on the other tubular. With the teeth alignment preserved, thefirst and second wellbore tubulars are then brought into a position suchthat their threads can be engaged by the threads of the collar and thecollar is rotated about its long axis to engage the tubular pin ends anddraw the pin ends into the collar. As the tubulars are drawn in by thecollar, the teeth become meshed at the thread crest.

Once threaded together, the interlock provided by the intermeshed teethact against, and may prevent completely, back off in the connection evenwhere there is considerable left hand torque. In addition, torque tendsnot to be transferred through the threads of the connection. Also, byallowing some tolerance between the pin end faces, the connection canallow for lateral flexing, such that the connection may not become toostiff.

When using the connection, it may be useful to position the right handthreaded pin end on the uphole end of the connection. Generally inwellbore operations, torque input from surface is most often to theright. As such, placing the right hand threaded pin end on the upper endof the connection ensures that even if the connection itself binds downhole, the string will in its normal rotation continue to drive the pinend into the connection, rather than backing off. That being said, it isbelieved that such a condition would be rare. It is believed that withthe pin to pin locking provided by the teeth, the only way the torquewon't transition through the connection is if the collar completelybinds down hole such that it cannot rotate, while the tubulars both haveopposite torque applied thereto sufficient to overcome the interlock ofthe teeth.

Left hand torque is common and often problematic in mud motorapplications such as the current motor driven drilling hammer. However,the connection may also be useful for other applications where left handtorque tends to act adversely on tubular connections such as in subsadjacent any rotationally driven drill bit.

To release the connection, the collar is reverse rotated about theconnection's axis .chi.t, again while the tubulars are held stationary.

The previous description of the disclosed embodiments is provided toenable any person skilled in the art to make or use the presentinvention. Various modifications to those embodiments will be readilyapparent to those skilled in the art, and the generic principles definedherein may be applied to other embodiments without departing from thespirit or scope of the invention. Thus, the present invention is notintended to be limited to the embodiments shown herein, but is to beaccorded the full scope consistent with the claims, wherein reference toan element in the singular, such as by use of the article “a” or “an” isnot intended to mean “one and only one” unless specifically so stated,but rather “one or more”. All structural and functional equivalents tothe elements of the various embodiments described throughout thedisclosure that are know or later come to be known to those of ordinaryskill in the art are intended to be encompassed by the elements of theclaims. Moreover, nothing disclosed herein is intended to be dedicatedto the public regardless of whether such disclosure is explicitlyrecited in the claims. No claim element is to be construed under theprovisions of 35 USC 112, sixth paragraph, unless the element isexpressly recited using the phrase “means for” or “step for”.

1-72. (canceled)
 73. A drilling accelerator comprising: a housingincluding an upper end and a lower end; a drive connection including anupper axially rotatable drive shaft for receiving an input of rotationalmotion, a rotational to axial mechanical drive converter incommunication with the upper axially rotatable drive shaft forconverting the input of rotational motion to an axial sliding motion; alower longitudinally moveable drive shaft in communication with therotational to axial mechanical drive converter to receive the axialsliding motion from the rotational to axial mechanical drive converterand a lower drill bit installation site connected to the lowerlongitudinally moveable drive shaft for receiving the axial slidingmotion and capable of conveying axial percussive motion there through,the lower drill bit installation site telescopically mounted adjacentthe lower end of the housing and slidably moveable relative thereto. 74.The drilling accelerator of claim 73 wherein the lower longitudinallymoveable drive shaft includes a drill bit box sub.
 75. The drillingaccelerator of claim 73 wherein the lower drill bit installation site isa drill bit box.
 76. The drilling accelerator of claim 73 wherein thelower drill bit installation site is connected for rotational movementwith the housing.
 77. The drilling accelerator of claim 73 wherein thelower drill bit installation site is connected for rotational movementwith the upper axially rotatable drive shaft.
 78. The drillingaccelerator of claim 73 wherein the rotational to axial mechanical driveconverter includes a gear assembly driving an eccentric member.
 79. Thedrilling accelerator of claim 78 wherein the gear assembly includesgears to convey rotational motion to the lower drill bit installationsite.
 80. The drilling accelerator of claim 73 wherein the rotational toaxial mechanical drive converter includes a cam assembly.
 81. Thedrilling accelerator of claim 80 wherein the cam assembly is in operableexcept when the lower longitudinally moveable drive shaft is drivenupwardly into the housing by weight on bit.
 82. The drilling acceleratorof claim 80 wherein the cam assembly includes cam surfaces that separateout of operational contact when the drilling accelerator is notpositioned with weight on bit.
 83. The drilling accelerator of claim 80wherein upper axially rotatable drive shaft is secured to move axiallywith the housing and the cam assembly includes an upper cam surface onthe housing and a lower cam surface on the lower longitudinally moveabledrive shaft and the upper cam surface of the housing is driven by thelower cam surface to lift the housing and the upper axially rotatabledrive shaft and drop the upper axially rotatable drive shaft onto thelower longitudinally moveable drive shaft to create the axial percussivemotion.
 84. The drilling accelerator of claim 73 wherein the upperaxially rotatable drive shaft and the lower longitudinally moveabledrive shaft are connected by a telescoping connection such that thelower longitudinally moveable drive shaft can slide axially relative tothe upper axially rotatable drive shaft.
 85. The drilling accelerator ofclaim 84 wherein the telescoping connection is configured to conveyrotational drive therethrough.
 86. The drilling accelerator of claim 73the housing includes a threaded connection connecting a first tubularsection of the housing and a second tubular section of the housing, thefirst tubular section including a first threaded pin end with a righthand thread form and a protrusion extending from its pin end face tocreate a stepped area thereon, the second tubular section of the housingincluding a second threaded pin end with a left hand thread form and arecess on its pin end face forming a shoulder sized to accept thestepped area of the first pin end seated thereagainst, and the threadedconnection including a collar having a first threaded box with a firstselected thread form selected to threadedly engage the right hand threadform of the first threaded pin end and a second threaded box with asecond thread form selected to threadedly engage the left hand threadform of the second threaded pin end.
 87. The drilling accelerator ofclaim 73 further comprising a positive displacement motor including astator housing and a rotor within the stator housing, the housingconnected at its upper end below and for movement with the statorhousing of the positive displacement motor and the rotor providing theinput of rotational motion to the drive connection and a drill bitconnected below the lower drill bit installation site.
 88. A wellborestring tubular connection comprising: a first tubular including a firstthreaded pin end with a right hand thread form and a protrusionextending from its pin end face to create a stepped region thereon, asecond tubular including a second threaded pin end with a left handthread form and a recess on its pin end face forming a shoulder sized toaccept the stepped region of the first pin end seated thereagainst, anda collar including a first threaded box with a first selected threadform selected to threadedly engage the right hand thread form of thefirst threaded pin end and a second threaded box with a second threadform selected to threadedly engage the left hand thread form of thesecond threaded pin end.
 89. The wellbore string tubular connection ofclaim 88 wherein the first tubular and the second tubular 388 arehousing sections of a mud motor-containing down hole assembly.
 90. Thewellbore string tubular connection of claim 88 wherein the protrusionincludes a left hand facing stepped region and a right hand facingstepped region and the recess includes a right hand facing shoulder anda left hand facing shoulder forming a gap therebetween sized to acceptthe protrusion.
 91. The wellbore string tubular connection of claim 88wherein the stepped region extends along a line substantially alignedwith the first tubular's long axis.
 92. The wellbore string tubularconnection of claim 88 wherein the stepped region creates a toothextending out from the first tubular pin end face and the recess forms agap in the second tubular pin end face and wherein the tooth is sized tofit closely into the gap.
 93. The wellbore string tubular connection ofclaim 88 wherein the first tubular includes a plurality of protrusionsand the second tubular includes a plurality of recesses and theplurality of protrusions of the first tubular, the plurality of recessesof the second tubular being selected to mesh together when the first andthe second tubulars are brought into pin-end to pin end contact toresist relative rotation between the first tubular and the secondtubular about their long axis.
 94. The wellbore string tubularconnection of claim 88 wherein the stepped region includes a face formedto extend substantially along a radial line extending out from a centeraxis of the first tubular.
 95. The wellbore string tubular connection ofclaim 88 wherein gaps are provided in the connection to provide forlateral flex.
 96. A method for making up a wellbore connection, themethod comprising: providing a first wellbore tubular with a threadedpin end and an tooth extending from a pin end face thereof, a secondtubular with a threaded pin end and recess in a pin end face thereof,the recess sized to accept the tooth of the first wellbore tubular and acollar including a first threaded box end and an opposite threaded boxend; aligning the first wellbore tubular and the second wellbore tubularto be threaded into the box ends of the collar and with the toothaligned with the recess and rotating the collar about its long axis toengage the threaded pin ends of the first wellbore tubular and thesecond wellbore tubular and draw the threaded pin ends into the collar.97. The method of claim 96 wherein the threaded pin end of the firsttubular includes a right hand thread form and the first threaded box endof the collar includes a thread form to accept the right hand threadform and the method further comprises, positioning the wellboreconnection with the first tubular on the uphole end.
 98. A percussionadapter comprising: a housing including an upper end and a lower end; adrive connection including an upper axially rotatable drive shaft forreceiving an input of rotational motion, a rotational to axialmechanical drive converter in communication with the upper axiallyrotatable drive shaft for converting the input of rotational motion toan axial sliding motion; a lower longitudinally moveable drive shaft incommunication with the rotational to axial mechanical drive converter toreceive the axial sliding motion from the rotational to axial mechanicaldrive converter and a base end of the lower longitudinally moveabledrive shaft for receiving the axial sliding motion and capable ofconveying axial percussive motion there through, the base end mountedadjacent the lower end of the housing and slidably moveable relativethereto.
 99. The percussion adapter of claim 98 wherein the lowerlongitudinally moveable drive shaft connects directly to the wellborestructure.
 100. The percussion adapter of claim 98 wherein the lowerwellbore structure installation site is connected for rotationalmovement with the housing.
 101. The percussion adapter of claim 98wherein the base end is connected for rotational movement with the upperaxially rotatable drive shaft.
 102. The percussion adapter of claim 98wherein the rotational to axial mechanical drive converter includes agear assembly driving an eccentric member.
 103. The percussion adapterof claim 102 wherein the gear assembly includes gears to conveyrotational motion to the lower wellbore structure installation site.104. The percussion adapter of claim 98 wherein the rotational to axialmechanical drive converter includes a cam assembly.
 105. The percussionadapter of claim 104 wherein the cam assembly is inoperable except whenthe lower longitudinally moveable drive shaft is driven upwardly intothe housing.
 106. The percussion adapter of claim 104 wherein the camassembly includes cam surfaces that separate out of operational contactwhen the percussion adapter is placed in tension.
 107. The percussionadapter of claim 104 wherein upper axially rotatable drive shaft issecured to move axially with the housing and the cam assembly includesan upper cam surface on the housing and a lower cam surface on the lowerlongitudinally moveable drive shaft and the upper cam surface of thehousing is driven by the lower cam surface to lift the housing and theupper axially rotatable drive shaft and drop the upper axially rotatabledrive shaft onto the lower longitudinally moveable drive shaft to createthe axial percussive motion.
 108. The percussion adapter of claim 98wherein the upper axially rotatable drive shaft and the lowerlongitudinally moveable drive shaft are connected by a telescopingconnection such that the lower longitudinally moveable drive shaft canslide axially relative to the upper axially rotatable drive shaft. 109.The percussion adapter of claim 108 wherein the telescoping connectionis configured to convey rotational drive therethrough.
 110. Thepercussion adapter of claim 98 wherein the housing includes a threadedconnection connecting a first tubular section of the housing and asecond tubular section of the housing, the first tubular sectionincluding a first threaded pin end with a right hand thread form and aprotrusion extending from its pin end face to create a stepped areathereon, the second tubular section of the housing including a secondthreaded pin end with a left hand thread form and a recess on its pinend face forming a shoulder sized to accept the stepped area of thefirst pin end seated thereagainst, and the threaded connection includinga collar having a first threaded box with a first selected thread formselected to threadedly engage the right hand thread form of the firstthreaded pin end and a second threaded box with a second thread formselected to threadedly engage the left hand thread form of the secondthreaded pin end.
 111. The percussion adapter of claim 98 furthercomprising a positive displacement motor including a stator housing anda rotor within the stator housing, the housing connected at its upperend below and for movement with the stator housing of the positivedisplacement motor and the rotor providing the input of rotationalmotion to the drive connection and a wellbore structure connected belowthe base end.
 112. A wellbore drilling percussion adapter comprising: anupper housing; a fluid flow channel extending from the upper end to thelower end of the upper housing, the first fluid flow channel including afluid entry end opening adjacent the upper end and a fluid exit adjacentthe lower end; a lower housing telescopically installed for axiallysliding motion relative to the lower end of the upper housing; a secondfluid flow channel extending from an inlet end open at the upper end toa discharge end opening at the lower end of the lower housing, the inletend being in fluid communication with the outlet end of the first fluidflow channel; an axial drive generator positioned to act with fluid flowthrough the first fluid flow channel and the second fluid flow channeland including a valve and a pressure chamber adjacent the valve, thevalve being closeable to create a force in the pressure chamber to drivethe upper housing axially away from the lower housing and the valvebeing openable to release pressure from the pressure chamber such thatthe upper housing can be forced down against the lower housing to createa hammering effect against the lower housing.
 113. The wellbore drillingpercussion adapter of claim 112 wherein the valve meters fluid passingfrom the first fluid flow channel through the second fluid flow channelto create a back pressure driving the upper housing longitudinallyoutwardly from the lower housing when the valve is closed and allowingthe parts to come together when the valve is opened, the valve beingopened by a back pressure when the fluid pressure exceeds a set valueand the valve being closed to create the back pressure when the pressurefalls below the set value.
 114. A method for applying a vibratory forceto a downhole string, the method comprising: providing a wellbore stringwith a percussion adapter installed therein, a fluid supply to thepercussion adapter and a drill bit installed below the percussionadapter; positioning the bottom hole assembly relative to a formation todrill a wellbore; pumping fluid through the percussion adapter togenerate axial percussive motion by generating a back pressure causingan upper housing of the percussion adapter to lift away from and a lowerportion of the percussion adapter and releasing the back pressure toallow the upper housing to drop onto the lower portion to create apercussive force, the percussive force being communicated to thewellbore string to create a vibration therein and; discharging fluidfrom the percussion adapter to continue to pass through the wellborestring.